The solubility of most ions in solution decreases with a decrease in temperature and pressure of the solution. If dissolved ions are present near their saturation concentration in the solution, a slight reduction in the temperature or pressure of the system can result in precipitation of a portion of these ions. Precipitates can combine and deposit as a scale on any solid surface with which they come into contact, such as the vessel or conduit in which the solution is confined.
One example of such a solution is a geothermal brine which is flashed at least in part to steam in a power plant or industrial process. This flashing is often accompanied by the formation of scale on the surfaces contacted by the fluid stream. Scale deposits tend to build up over a period of time and restrict further fluid flow. In extreme cases, conduits can become completely plugged with scale and the industrial operation must be shut down for maintenance.
Typically, salts and oxides of heavy metals, such as lead, zinc, iron, silver, cadmium and molybdenum, are found in geothermal brine. Other more common minerals, such as calcium and sodium, are also dissolved in the brine, as are naturally occurring gases, including carbon dioxide, hydrogen sulfide and methane.
An especially troublesome component of the hot brine may be silica, which may be found near saturation concentrations in the form of silicic acid oligomers. These tend to precipitate out at almost every stage of brine processing, either as substantially pure silica or as a tightly adherent metal-silica scale. Silica rich scale/precipitation, especially that which forms at lower temperatures, has little or no commercial value because little of the valuable metals are present. Typically, this "natural" precipitation from temperature and pressure reduction removes less than one percent by weight of the valuable metals present. If this troublesome "natural" scale is not removed, or prevented from forming, serious problems can arise with conduit plugging. Even when the brine has completed its passage through the plant, it typically retains some amount of scale/precipitation forming tendencies, which if not removed, will eventually result in the plugging of the injection wells used to return the now cooled brine to the geothermal field.
Various proposals have been made to decrease the scale formation in equipment used in producing and handling geothermal brine. In "Field Evaluation of Scale Control Methods: Acidification," by J.Z. Grens et al, Lawrence Livermore Laboratory, Geothermal Resources Council, Transactions, Vol. 1, May 1977, there is described an investigation of the scaling of turbine components wherein a geothermal brine at a pressure of 220 to 320 p.s.i.g. and a temperature of 200.degree. to 230.degree. C. (392.degree. to 446.degree. F.) was expanded through nozzles and impinged against static wearblades to a pressure of 1 atmosphere and a temperature of 102.degree. C. (215.degree. F.). In the nozzles, the primary scale was heavy metal sulfides, such as lead sulfide, copper-iron sulfide, zinc sulfide and cuprous sulfide. Thin basal layers of fine-grained, iron-rich amorphous silica appeared to promote the adherence of the primary scale to the metal substrate. By contrast, the scale formed on the wearblades was cuprous sulfide, native silver and lead sulfide in an iron-rich amorphous silica matrix. When the brine which originally had a pH of 5.4 to 5.8 was acidified with sufficient hydrochloric acid to reduce the pH of the expanded brine to values between 1.5 to 5.0, scaling was eliminated. However, such acidification of hot brines increases the corrosion of the brine-handling conduits and equipment.
It also is known to recover metal values and salts from brine, such as geothermal brine produced from a subterranean reservoir. U.S. Pat. No. 4,127,989 to Michelson discloses a method in which brine is pressurized and maintained above the bubble point pressure and thereafter a precipitating agent, such as a soluble sulfide, is added to the brine to enhance formation of insoluble metal sulfide precipitates. Soluble salts and metal values are recovered from the brine effluent after the hot brine has been processed to recover energy therefrom. Silver sulfides are among the mineral values recovered by this process.
Moreover, the treated brine must be particle-free after the metal recovery process for injection into the reservoir. In addition, the treated brine must be compatible with the reservoir, e.g., the treated brine must not adversely react with the reservoir formation when the brine is injected. This may render a precipitating agent addition metal recovery process unsuitable for application to geothermal brine which is returned to the resevoir formation, unless the precipitating) agent is neutralized prior to injection. Neutralization may require costly amounts of reagents, process materials, and equipment, such as pH control and filtration of the entire flow of brine.
Still further, treatments can cause corrosion or other fluid handling problems. Other problems can include the introduction of oxygen (e.g., along with the precipitating agent) into the otherwise oxygen-free brine, contamination of heat recovery processes, and embrittlement of equipment.
While the aforementioned geothermal brine treatments have met with some success in particular applications, the need exists for a further improved treating process to better control the scaling during (thermal) energy recovery and injection processes, and to efficiently recover valuable mineral values in geothermal brines.
Accordingly, it is the object of this invention to provide an improved method for recovering valuable precious metal-containing scales from geothermal brines, inhibiting the overall precipitation of scale, particularly iron-silicate scale, inhibiting corrosion, and polishing the geothermal fluid so as to prevent the transport of residual silica and other particulate material suspended therein to an injection well.
Other objects, advantages and features of the invention will be apparent from the following description, drawings and appended claims.